The invention relates to a projection lens for imaging a pattern arranged in an object plane of the projection lens into an image plane of the projection lens with electromagnetic radiation having a working wavelength λ from the extreme ultraviolet range (EUV). Furthermore, the invention relates to a film element provided, in particular, for use in such a projection lens, and to a method for producing a projection lens containing a film element.
Nowadays predominantly microlithographic projection exposure methods are used for producing semiconductor components and other finely structured components. In this case, use is made of masks (reticles) or other patterning devices which carry or form the pattern of a structure to be imaged, e.g. a line pattern of a layer of a semiconductor component. The pattern is positioned in a projection exposure apparatus between an illumination system and a projection lens in the region of the object surface of the projection lens and illuminated with an illumination radiation provided by the illumination system. The radiation altered by the pattern passes as projection radiation through the projection lens, which images the pattern onto the substrate to be exposed, which is generally coated with a radiation-sensitive layer (resist, photoresist).
In order to be able to produce ever finer structures, in recent years projection lenses have been developed which operate with moderate numerical apertures and achieve an increase in the resolution capability substantially using the short wavelengths of the used electromagnetic radiation from the extreme ultraviolet range (EUV). In particular, wavelengths in the range of between 5 nm and 30 nm are used here.
Radiation from the extreme ultraviolet range (EUV radiation) cannot be sufficiently focused or guided with the aid of refractive optical elements, since the short wavelengths are greatly absorbed by the known optical materials that are transparent at higher wavelengths, or other materials. Therefore, mirror systems are used for EUV lithography. A mirror (EUV mirror) having a reflective effect for radiation from the EUV range typically has a substrate, on which is applied a multilayer arrangement having a reflective effect for radiation from the extreme ultraviolet range and having a large number of layer pairs comprising alternately relatively low refractive index and relatively high refractive index layer material and acting in the manner of a distributed Bragg reflector. Layer pairs for EUV mirrors are often constructed with the layer material combinations molybdenum/silicon (Mo/Si) and/or ruthenium/silicon (Ru/Si).
An EUV projection lens comprises a plurality of mirrors, e.g. four or six mirrors, having mirror surfaces which are arranged in a projection beam path between the object plane and the image plane in such a way that a pattern arranged in the object plane can be imaged into the image plane using the mirrors in a manner as free from aberrations as possible. The rays of a projection beam that run between the object plane and the image plane form a wavefront. Deviations of the wavefront from a wavefront predefined by the specification can lead to imaging aberrations that cannot be afforded tolerance.
Projection lenses for EUV lithography demand very precise manufacture of the optical elements and a precise coating. What is problematic in this case is, inter alia, that the true state of the optical elements (in particular owing to the coating) can be measured sufficiently precisely only in the assembled state at the working wavelength (e.g. 13.5 nm). In this stage, often all that remains for a subsequent correction is very complex partial disassembly of the projection lens in order to rework mirrors. Beyond rigid-body movements are hardly any concepts that function in a sustained manner for the correction of lifetime effects.